General Relativistic Spectra of Accretion Disks around Rotating Neutron Stars
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General relativistic spectra from accretion disks around rotating neutron stars in the appropriate spacetime geometry for several different equations of state, spin rates, and masses of the compact object have been computed. The analysis involves the computation of the relativistically corrected radial temperature profiles and the effect of Doppler and gravitational redshifts on the spectra. Light-bending effects have been omitted for simplicity. The relativistic spectrum is compared with the Newtonian one, and it is shown that the difference between the two is primarily a result of the different radial temperature profiles for the relativistic and Newtonian disk solutions. To facilitate direct comparison with observations, a simple empirical function has been presented which describes the numerically computed relativistic spectra well. This empirical function (which has three parameters including normalization) also describes the Newtonian spectrum adequately. Thus, the function can in principle be used to distinguish between the two. In particular, the best-fit value of one of the parameters (β-parameter) ≈0.4 for the Newtonian case, while it ranges from 0.1 to 0.35 for the relativistic case depending upon the inclination angle, equation of state (EOS), spin rate, and mass of the neutron star. Constraining this parameter by fits to future observational data of X-ray binaries will indicate the effect of strong gravity in the observed spectrum.Keywords:
Relativistic quantum chemistry
Relativistic beaming
Compact star
The recent observation of the object HESS J1731-347 suggests the existence of a very light and very compact neutron star being a challenge for commonly used equation of state for dense matter. In this work we present a relativistic mean field model enriched with meson crossing terms among isovector and isoscalar mesons. Such interactions particularly dominate the behavior of the symmetry energy and accounts for small size of compact star radius. The proposed model fulfill the recent constraints concerning the symmetry energy slope and state-of-the-art compact stars constraints derived from the NICER measurements of PSR J0030+0451 and PSR J0740+6620 pulsars as well as from the GW170817 event and its associated electromagnetic counterparts AT2017gfo/GRB170817A.
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Recent observational and theoretical results suggest that Short-duration Gamma-Ray Bursts (SGRBs) are originated by the merger of compact binary systems of two neutron stars or a neutron star and a black hole. The observation of SGRBs with known redshifts allows astronomers to infer the merger rate of these systems in the local universe. We use data from the SWIFT satellite to estimate this rate to be in the range $\sim 500$-1500 Gpc$^{-3}$yr$^{-1}$. This result is consistent with earlier published results which were obtained through alternative approaches. We estimate the number of coincident observations of gravitational-wave signals with SGRBs in the advanced gravitational-wave detector era. By assuming that all SGRBs are created by neutron star-neutron star (neutron star-black hole) mergers, we estimate the expected rate of coincident observations to be in the range $\simeq 0.2$ to 1 ($\simeq 1$ to 3) yr$^{-1}$.
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Moment of inertia
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The first detections of afterglows from short gamma-ray bursts (GRBs) have confirmed the previous suspicion that they are triggered by a different central engine than long bursts. In particular, the recent detections of short GRBs in galaxies without star formation lends support to the idea that an old stellar population is involved. Most prominent are mergers of either double neutron stars or of a neutron star with a stellar-mass black hole companion. Since the final identification of the central engine will only come from an integral view of several properties, we review the observable signatures that can be expected from both double neutron stars and neutron star black hole systems. We discuss the gravitational wave emission, the structure of the neutrino-cooled accretion disks, the resulting neutrino signal and possible mechanisms to launch a GRB. In addition, we address the speculative idea that in some cases a magnetar-like object may be the final outcome of a double neutron star merger. We also discuss possibilities to explain the late-time X-ray activity that has been observed in several bursts.
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Abstract To constrain the equation of state of cold dense matter, astrophysical measurements are essential. These are mostly based on observations of neutron stars in the X-ray band, and, more recently, also on gravitational wave observations. Of particular interest are observations of unusually heavy or light neutron stars which extend the range of central densities probed by observations and thus permit testing nuclear physics predictions over a wider parameter space. Here we report on the analysis of such a star, a central compact object within the supernova remnant HESS J1731-347. We estimate the mass and radius of the neutron star to be M = 0.77(20)Msun and R = 10.4(8) km, respectively, based on modeling of the X-ray spectrum and a robust distance estimate from Gaia observations. Our estimate implies that this object is either the lightest neutron star known, or a "strange star” with a more exotic equation of state. Adopting a standard neutron star matter hypothesis significantly constrains the corresponding equations of state.
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Abstract I discuss the nature of the compact X-ray source inside the supernova remnant RCW 103. Several models, based on the accretion of matter onto a compact object such as a neutron star or a black hole (isolated or binary), are analysed. I show that it is more likely that the X-ray source is an accreting neutron star than an accreting black hole. I also argue that models of a binary system with an old accreting neutron star are most favoured.
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Compact objects like stellar-mass black holes and neutron stars are dense enough to significantly warp spacetime. By studying emission from very close to the compact object, we can decipher the effects of strong-gravity on physical processes, and test general relativity in the strong-field limit. There is a plethora of rapid sub-second variability in the X-ray light curves from compact objects in accreting low-mass X-ray binaries (LMXBs), and a growing toolbox of analysis techniques and algorithms to apply to such phenomena. This thesis investigates quasi-periodic oscillations (QPOs) and coherent X-ray burst oscillations. QPOs are a probe for studying physical processes in the inner regions of LMXBs, and X-ray burst oscillations are used to determine the masses and radii of neutron stars to constrain the neutron star equation of state. In Chapter 2, we present a novel spectral-timing technique to do phase-resolved spectroscopy of QPOs that tracks the variations of spectral parameters with QPO phase, and we apply it to a low-frequency QPO from the black hole GX 339-4. In Chapter 3, we use ray-tracing to simulate pulse profiles of thermonuclear burst oscillations from an accreting neutron star and fit these with an evolutionary optimization algorithm. In Chapter 4, we apply our phase-resolved spectroscopy technique to a lower kilohertz (kHz) QPO from the neutron star 4U 1608-52. Finally in Chapter 5, we carry out spectral-timing analysis of the low-frequency QPO seen by NICER in the new black hole transient MAXI J1535-571.
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The discovery of non-diffuse sources of gravitational waves through compact-object mergers opens new prospects for the study of physics beyond the Standard Model. In this Letter, we consider the implications of the observation of GW190814, involving a coalescence of a black hole with a $\sim$2.6 $M_\odot$ compact object, which may be too massive to be a neutron star, given our current knowledge of the nuclear matter equation of state. We consider the possibility of a new force between quarks, suggested in other contexts, that modifies the neutron star equation of state, particularly at supranuclear densities. We evaluate how this modification can impact a neutron star's mass and radius to make the observed heavy compact object more probably a neutron star, rather than a black hole, and suggest that further such objects may yet be found. We note the terrestrial and astrophysical measurements that could confirm our picture.
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